Junk Yard Genius Tools How-To...
The Vacuum Gauge


You Bet Your Life It IS!!

The entire combustion process demands continuous high and low pressure cells to operate.
A vacuum gauge lets us 'see' the invisible low pressure cells inside the engine, and with a good working knowledge of a few basic principals of nature, we can figure out what is wrong with an engine.

This is mostly for those of you with stock engines, or engines that have been slightly modified, which is most of you...

1. Vacuum readings are mandatory to tune ANY carburetor.
The most tuneable carburetor on the market is a Holley, and you will learn to live and die by the vacuum gauge tuning one of them.

2. Nearly every fuel injection system has a manifold pressure sensor.
That Manifold Absolute Pressure (MAP) sensor, combined with input from other sensors, determines fuel delivery volume and ignition advance in modern vehicles.

3. Many millions of transmissions have vacuum inputs.
Those inputs can be tuned by knowing what the vacuum is doing when the transmission is supposed to be shifting, and how to change the input of the vacuum information into the transmission.

4. Nearly every single older ignition system has a vacuum advance canister.
It is important to know what the vacuum in your engine is doing when the distributor advance curve is programmed into the distributor.

5. Knowing what your 'Normal' engine vacuum is supposed to be is really the key in finding problems easily with a vacuum gauge.
Taking vacuum readings, especially idle vacuum, is real easy, takes virtually no time, and will be a life saver when something goes wrong.
Taking regular vacuum reading at idle and through the RPM range up to about 2,500 RPM is the ONLY way to know exactly what is 'Normal' for your vehicle, especially if you have modified the engine in ANY way...

Use an UNDAMPENED or UNDAMPED vacuum gauge.
None of the fancy and expensive 'buffered' or oil filled gauges, they are worthless for our purposes.
Most of the better vacuum gauges intended for automotive service are satisfactory.
Try to get a quality gauge, Sears or better. A cheap gauge can not only miss a lot of things, but can give false readings.

(paraphrased from a rebuild manual)
1. A steady reading between 16 and 22 in.Hg. at idle...
This is normal for most vehicles.
Radically cammed engines have lower, less steady readings.

2. Normal range at idle, with sporadic drops below normal,
This could indicate a sticking valve.

3. Normal range at idle with needle vibration of about 2 in.Hg.,
This could indicate an ignition problem. Check plug gap, dwell (yes, electronic ignitions have dwell too), cap, rotor, and plug wires.

4. A steady reading slightly higher than normal.
This can be caused by a dirty air filter, or overly advanced ignition timing.

5. A steady reading 3 to 12 in.Hg. lower than normal.
This could indicate one or more of the following conditions,
Intake or carburetor vacuum leak, Late ignition or cam timing, Worn piston rings.

6. Gauge needle drifts slowly over a range of 4 to 5 in.Hg. at idle.
This could indicate an idle mixture that is too rich or too lean.

7. Gauge needle fluctuates rapidly between 10 and 21 in.Hg. at idle.
This could occur when one or more valve springs are weak or broken.

8. Gauge reads normal at idle, but drops slowly as engine speed is increased to 2,500 RPM.
This could indicate a restricted exhaust system.

9. Gauge reads below normal and fluctuates rapidly over a range of about 3 in.Hg. at idle, then the needle becomes steady as engine speed is increased.
Worn intake guides usually cause this reading.

As you can see, a vacuum gauge is a mandatory piece of diagnostic equipment, and absolutely mandatory for tuning a carburetor.
If you intend on buying any diagnostic or tuning equipment, a good quality vacuum gauge should be at the top of your list.